Reticles or masks are comprised of a patterned opaque coating such as chrome on a transparent substrate and are used to manufacture integrated circuits for semiconductor devices, MEMS devices, or other devices requiring the formation of small features on a substrate. High quality reticles are quite valuable because of the time and expense needed to produce them. The time required to build a reticle can take up to two weeks and the cost can easily be $20,000 or more because expensive electron beam tools are used to form patterns and expensive focused ion beam tools may be employed to repair defects.
Reticles are usually fabricated by forming a pattern in a photoresist film on an underlying opaque coating on a substrate. The pattern in the photoresist is transferred through the underlying opaque coating by an etch step. The photoresist serves as an etch mask and regions which are clear of photoresist before etch become regions clear of opaque coating after etch. The amount of opaque coating remaining on the substrate after the etch transfer can vary from a few % to nearly all of the original surface area. One of the most common substrates is quartz which is transparent to wavelengths in the range of 190 nm to 600 nm that are used to expose photoresist films on substrates during the manufacture of integrated circuits.
During the manufacture of integrated circuits, reticles must be protected from particles that might adhere to its surface: A covering consisting of a thin film called a pellicle is mounted on a frame or spacer to connect the pellicle to a reticle but leaving an air gap between the reticle and pellicle. Dust particles are kept off the reticle by collecting on the pellicle which is typically a thin film of highly transparent material such as nitrocellulose. Any particle that collects on the pellicle does not affect the aerial image of the pattern transmitted from the reticle onto the photoresist film on a device substrate. Since the particle is not in the focal plane on the reticle surface, its image is not reproduced on the photoresist film. Otherwise, any particle that collects on a clear quartz region on a reticle is reproduced as a defect in the photoresist film and can eventually degrade the performance of the resulting device. U.S. Pat. No. 5,577,610 describes an anti-static casing to hold frame supported pellicles and keep particles from collecting on pellicle surfaces. The casing only touches the pellicle frame at a few points while it is in storage and is not useful for dissipating electrostatic charges that can form on a reticle while it is connected to a pellicle by a frame in an exposure tool or in transit between an exposure tool and storage.
Occasionally, pellicles must be replaced because they become torn during handling or because repeated exposure causes light transmission through them to decrease to an unacceptable level. A decrease in transmission means the exposure tool will have to be used for a longer period to expose each substrate with a predetermined dosage level needed to pattern a photoresist. This is costly because the throughput of the production line will be reduced to give fewer devices per unit time.
Besides particle defects, another big concern associated with handling reticles is electrostatic charge build up on the reticle which if not readily dissipated can cause electrostatic damage to the reticle. The quartz substrate is an insulator and therefore electrostatic charges can easily build up on the surface. A charge build up on the metallic chrome coating can also occur. Small quartz spaces between two chrome features are especially susceptible to damage. For example, an electrical bridge is likely to form in a small quartz space between the ends of two chrome features on a reticle as depicted in FIGS. 4a-4d. Damage to the quartz can lead to a loss of transmission through the affected area and the pattern transferred into the photoresist film in a subsequent device patterning step will not be reproduced accurately from the mask design. Instead, the chrome regions next to the damaged quartz will appear as oversized photoresist features that may bridge together because the intensity of the light through the damaged quartz is not sufficient to form an opening in the photoresist film as intended.
Electrostatic charges can also form on the pellicle film which is an insulator and these charges can migrate to the reticle. Therefore, it will be an advantage to have a method of preventing charge build up on a reticle that is also effective in avoiding charging on pellicle surfaces.
In prior art, anti-static coatings have been applied to reticles during the reticle fabrication process to prevent charge build up during pattern formation with an electron beam tool. U.S. Pat. No. 5,533,634 mentions the use of alumina as an anti-static layer which is etched and stripped prior to the end of the mask making process. Similarly, U.S. Pat. No. 5,482,799 describes the use of a molybdenum film for charge dissipation on a substrate during a mask making process. Again, the conductive film is sacrificial and does not remain on the reticle when it is used in a manufacturing line to fabricate integrated circuits.
Another example in which a conductive film is applied to a reticle to prevent charge build up is found in U.S. Pat. No. 5,357,116. A layer of an ammonium salt is coated on a reticle before a focused ion beam (FIB) repair step to allow an accurate placement of the beam on the substrate. The conductive layer is also useful for avoiding charging during processing with an electron beam exposure. The preferred thickness is about 1 micron to provide adequate step coverage over chrome lines. The layer is removed at the end of the fabrication process and does not protect the reticle once it is released to a device manufacturing line.
Others skilled in the art have produced reticles in which a light absorbing material is formed inside a transparent substrate rather than on a substrate surface. The structure described in U.S. Pat. No. 5,474,865 does not require a pellicle and can prevent electrostatic damage to quartz between the opaque components inside the substrate. However, the process of producing and repairing the reticle would be quite expensive and make it unattractive for a low cost device manufacturing process.
Therefore, an improved method of protecting a reticle from charge build up and damage to its quartz substrate is needed. In particular, the protective material should be effective during the entire time the reticle is in a manufacturing environment. The protective method should ideally be capable of dissipating charges from the reticle and pellicle while they are in an exposure tool, in storage, or in transit between an exposure process and a storage mode. The method should be cost effective and easy to implement in a manufacturing line.